Heretofore, encrypted digital contents have been transmitted to user terminals such as PCs (Personal Computers), cellular phones, and so forth. Note that examples of digital contents include moving images, music, books, and dictionaries. Also, examples of a reason for encrypting digital contents include a reason for protecting a copyright.
Also, there are apparatuses which subject data to data compression using a DPCM (Differential Pulse Code Modulation) method, and encrypt the data using an encryption algorithm or encryption key which differs depending on code representing difference 0 and code representing other than difference 0 in the DPCM method.
Also, there are apparatuses which encrypt data by dividing the data into a plurality of data, and encrypt the plurality of data using multiple encryption methods respectively.
Though decryption difficulty as to encrypted data differs depending on encryption algorithms, even an encryption algorithm having decryption difficulty may be costly in encryption processing. Conversely, even an encryption algorithm that is costly in encryption processing may readily be decrypted.